Nanomaterials are materials that include components with nanometer dimensions, for example, where at least one dimension is less than 100 nanometers. Examples of such materials are allotropes of carbon such as nanotubes or other carbon fullerenes and components of carbon char. Carbon black was an early use of nanomaterials in tire manufacturing. Other nanomaterials include inorganic materials such as metal sulfides, metal oxides and organic materials. Because of the small dimensions, nanomaterials often exhibit unique electrical and electrochemical properties and unique energy transport properties. These properties are most pronounced when high surface areas are present and when charge transport mechanisms exist in the nanomaterials.
Some nanomaterials are manufactured using rigorous processing steps that are expensive and commercially unattractive. Some nanomaterials occur naturally or incidentally in commercial processing steps. Naturally or incidentally occurring nanomaterials tend to be highly irregular in size and composition because the environment in which they are produced is not adequately controlled for the production of nanomaterials. Processing methods that produce nanomaterials include among others, liquid-phase steps, gas-phase steps, grinding steps, size-reduction steps and pyrolysis steps.
Pyrolysis is the heating of materials in the absence of oxygen to break down complex matter into simpler molecules and components. When carbon based materials are pyrolyzed, the process of carbonization can occur leading to an ordered state of semi-graphitic material. When carbon based materials are pyrolyzed in uncontrolled conditions, a large amount of randomly ordered carbon material results. When both carbon and inorganic materials are present, pyrolysis under controlled conditions can lead to highly useful and unique results. An example of a use of pyrolysis is for the break down of used tires (typically from automobiles, trucks and other vehicles). The pyrolysis of tires results in, among other things, a carbon/inorganic residue called char.
The composition of char from tire pyrolysis is determined by the materials that are used to manufacture tires. The principal materials used to manufacture tires include rubber (natural and synthetic), carbon black (to give strength and abrasion resistance), sulfur (to cross-link the rubber molecules in a heating process known as vulcanization), accelerator metal oxides (to speed up vulcanization), activation inorganic oxides (principally zinc oxide, to assist the vulcanization), antioxidant oxides (to prevent sidewall cracking), a textile fabric (to reinforce the carcass of the tire) and steel belts for strength. The carbon black has a number of carbon structures including graphitic spheroids with nanometer dimensions, semi graphitic particles and other forms of ordered carbon structures.
In summary, the manufacture of tires initially mixes the materials to form a “green” tire where the carbons and oxides form a homogenous mixture. The “green” tire is transformed into a finished tire by the curing process (vulcanization) where heat and pressure are applied to the “green” tire for a prescribed “cure” time. The carbon materials used in “green” tires are typically as indicated in TABLE 1:
TABLE 1DESIGNATIONSIZE (nm)N11020-25N22024-33N33028-36N30030-35N55039-55N68349-73
When tires are discarded, they are collected for pyrolysis processing to reclaim useful components of the tires. In general, tire pyrolysis involves the thermal degradation of the tires in the absence of oxygen. Tire pyrolysis has been used to convert tires into value-added products such as pyrolytic gas (pyro-gas), oils, char and steel. Pyrolysis is performed with low emissions and other steps that do not have an adverse impact on the environment. The basic pyrolysis process involves the heating of tires in the absence of oxygen. To enhance value, the oils and char typically under go additional processes to provide improved products.
In electrochemical capacitors, electrical charge is stored on the surface of an electrically conductive electrode material. The capacitance arises by separation of electrons at the electrode surface and ionic charges in the electrolyte solution. Because the charge separation arises over only a distance of 0.1 to 10 nanometers, large specific capacitances can be achieved on the order of 10-20 microfarads per square centimeter of electrode material. The larger the surface area of the electrode material, the greater the charge that can be stored. Since the capacitance, or the amount of charge that an electrochemical capacitor can hold, is directly related to the surface area of the electrodes, electrodes made from conductive materials with high surface areas are preferred. Devices incorporating such electrodes are referred to as double layer capacitors or supercapacitors.
Electrochemical capacitors are charge-storage devices that are capable of delivering high power densities and that are capable of being cycled (charged and discharged) millions of times, hence demonstrating a significant advantage over conventional batteries. Electrochemical capacitors have energy and power capabilities that lie between the capabilities of a battery and of a conventional capacitor (electrolytic, thin film and others).
There is substantial demand for a rechargeable energy source that can provide high power and energy densities, can be charged quickly, has a high cycle life is environmentally benign and cost effective. Double layer capacitors, especially when used in conjunction with batteries, are rechargeable charge storage devices that fulfill this need.
In prior art capacitors, the production of activated carbon is an energy intensive process that first includes heating of a precursor material (natural or synthetic) to form a carbon powder or carbon fiber, in many cases requiring temperatures up to 3000° C. Next, to form activated carbon, the material is heated to about 800° C. in an atmosphere of steam or carbon dioxide, or electrochemical reaction in a strongly oxidizing solutions (such as Hummers reagent) to produce a carbon with high surface area to provide high energy density and high power density. Overall, the yield for activated carbons is generally not better than 25% based on weight of the precursor material.
A single cell double-layer capacitor consists of two electrodes which store electrical charge (called the active materials), separated by an ion permeable but electrically insulating membrane. Each electrode is also in contact with a current collector which provides for electrical contact outside of the cell. The electrodes and membrane are infused with an electrolyte and enclosed in an inert housing which provides a sealed environment and also enough compression to reduce contact resistance between the different layers. Multiple cells may be used in series to increase the allowable potential (voltage), and also in parallel to increase the capacitance.
Applying an electrical potential across the electrodes causes charge to build up in the electrochemical double layer that exists at the electrode/electrolyte interface for each electrode. This process continues until a state of equilibrium is reached, so that the potential of the electrodes is at the charging potential and the current is reduced to that required to maintain the charge.
Because carbon is relatively chemically inert, has a high electrical conductivity, is environmentally benign, and is relatively inexpensive, some forms of carbon are excellent materials for fabricating electrodes. However, many forms of carbon are not suitable for electrodes. The desired properties of the electrochemical capacitor electrodes include the following high surface area, electrically conductive, low cost, readily available source of material and long-term stability under operating conditions.
Advances are being made in electrochemical capacitor technology research using nanomaterials. While capacitors of many types are known, there is a need for improved electrodes based on nanomaterials and for new electrochemical capacitors using the new nanomaterials.